For exothermic reactions (e.g., polyethylene (PE) and polypropylene (PP) polymerization) in fluidized bed reactors many benefits are gained by maximizing cooling on both macroscopic and microscopic levels. Macroscopic in this case implies the total volume of the reactor whereas microscopic refers to the immediate vicinity around the catalytic sites where polymerization is taking place. When cooling improves, catalyst productivity and polymer properties often improve. At the same time, reactor operability becomes more robust since hot spotting, sheeting, and chunking are reduced.
One way of maximizing cooling throughout the reaction zone, and particularly on the microscopic level is by evaporative cooling. To this end, polymerization processes in which a certain portion of the cycle gas stream is condensed and fed to the reactor as a liquid where it vaporizes have been used (i.e., "condensing mode").
Condensing mode processes have been limited due to several factors including only an estimate of the amount of liquid entering the bed was controlled/monitored. Thus, depending on other process parameters, the instantaneous rate of liquid evaporation in the reaction zone was variable and was not controlled. The inability to independently control rate of liquid evaporation can cause oscillation/variation in reaction control variables like reactor pressure, temperature, bed weight, and fluidized bulk density. This in turn can cause variations in product properties and catalyst productivity. Additionally, the amount of liquid entering the bed has been estimated using the assumption of thermodynamic equilibrium. Thus, prior art methods have not been able to account for the dynamic nature of the fluidized bed reaction system.